1. Field of the Invention
The present invention relates to the fabrication of optical fiber for signal transmission and more particularly, to an indirect heat type double-clad crystal fiber fabrication method.
2. Description of the Related Art
There are known many methods intensively used for making glass fibers. In a popularly known method, a glass cylinder is set in the top of a drawing tower, a high-temperature heating stove is used to indirectly heat the glass cylinder to a softened state, and then the softened glass cylinder is drawn downwards into a glass fiber. A glass fiber made according to this method has the advantages of low transmission loss, uniform diameter and rapid growth. However, this method is not practical for making single-crystal optical fiber because rapid growth results in a poor crystal quality.
There is also known laser heated pedestal growth method. This method eliminates the drawback of a drawer tower of being unable to grow single-crystal optical fiber, and also overcomes semiconductor manufacturing process's problem of being unable to be fiberized. A double-clad crystal fiber made subject to laser heated pedestal growth method has optimal mechanical characteristics, sufficient to bear watt grade pumping power and fusible with glass fiber to constitute a full-optical network system. Further, a double-clad crystal fiber nowadays can be made having the core as small as 20 microns and the fiber length as long as 6 cm. When pumped at wavelength 1064 nm and power 1.16 watt, it enables signal light of wavelength 1047 nm to produce 16 dB signal gain. If the core can be reduced to 10 microns, more than 30 dB signal gain could be obtained.
However, due to the power variation problem (±5%) of laser heated pedestal growth method, it is not easy to heat a glass cylinder directly with a laser beam for growing a double-clad optical fiber having the core smaller than 20 microns while maintaining the diameter in uniformity.